A range of other data and findings will also be available and
factored into the equation.  To summarize preliminary
findings:
Weather conditions are playing a dominant role, both
short and long term.
There are strong blooms of phytoplankton in the
epilimnion in the spring.
6.2
The hypolimnion is biologically very active.
There are dramatic changes in benthic processes in
both shallow and deep waters.
Possible key drivers include:
climate change;
and to determine the direction of future research that may
consequences of habitat degradation, for example, as a
be required.  The questions and preliminary findings to
result of changes in land use, shoreline hardening, and
date are the following.
wetlands; and
alien invasive species.
Possible reasons for expecting reduced thickness and /
or increased persistence of the hypolimnion:
Scientists are carefully exploring and considering the
warmer than normal summers (warmer surface water)?
evidence to establish linkages and determine which are
longer than normal summers (more time for oxygen
controlling or contributing factors to the observed changes
depletion)?
in Lake Erie.  Discussions took place at the Lake Erie Millen-
greater heating due to clearer water?
nium Conference at the University of Windsor, Ontario on
greater penetration by ultraviolet light (ozone deple-
May 6-7 [and at the International Association for Great Lakes
tion)?
Research conference in Chicago, Illinois on June 22-26].
Conclusions and advice will be available late in 2003.
Possible limits on primary production (phytoplankton
biomass):
Another observation is that changes in Lake Erie began about
high grazing pressure (especially by zebra and quagga
1996.  Before that date, ecosystem quality was improving, as
mussels)?
measured by phosphorus loadings and water concentrations,
nutrient limitations (phosphorus or nitrogen) in sub-
and the size of the anoxic zone.  However, the changes were
surface epilimnetic water?
not recognized until 1999 in part because of natural spatial
trace metal (iron or copper) limitations?
and temporal variability of the very dynamic Lake Erie
ultraviolet light- and contaminant-induced inhibition?
ecosystem, the reduced and intermittent sampling and
monitoring programs, and human nature (i.e. recent
Increases in phosphorus loadings?  Recent calculations
changes are most vivid).
suggest minimal increases from point sources;
however, there is uncertainty associated with the
loadings because of:
reduced data for tributaries (the result of cutbacks in
Because  of  its  natural  characteristics,  the  central
monitoring programs);
basin of Lake Erie has always experienced occasional
higher (less sensitive) detection limits being used in
reporting sewage treatment plant inputs; and
anoxic  problems,  the  severity  of  which  is
unusually strong autumn storms in recent years, which
determined by the thickness of the hypolimnion.
could result in strong, unmeasured pulses of phospho-
rus, due to increased land runoff and / or erosion and
Weather -- temperature and storms -- determine
resuspension of lake sediments.
the thickness.  The size and persistence of the “dead
Food web disruption and increased rate of nutrient
zone”  cannot  be  used  to  gauge  the  success  or
cycling or new pathways for cycling, because the
failure of phosphorus load reduction policies and
presence of zebra mussels may have disrupted the
programs introduced in the 1970s.
food web?
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